Costco for Flank Steak. You can get it at Whole Foods but it is twice as expensive per pound. As for Skirt Steak, I go to Whole Foods. Btw - Alton Brown has a great marinade for Skirt Steak.

http://www.foodnetwork.com/recipes/alton-brown/skirt-steak-recipe.html

I don't use his cooking method although I am going to try it one day. I just get my grill as hot as possible, 2-3 minutes a side depending on how you like your meat cooked.